Origin: Singlish/Sound- Used when calling out to someone (informal)
Oei or Oui is used when you call out to someone that you know. It is an informal way and some regard it as a being rude especially when you are the one being called out to.
David: Oei!! Long time no see how have you been??
Amy: Who are you? I dunno you..
David: Oei…..dun like that leh??
Amy: Oei…Oui….I don’t have name is it??? You obviously have forgotten!!!